Serial printers employing a wire-dot print head can be used to print on a variety of print media, such as multi-ply print papers, and they are used extensively. The wire-dot print heads drive wires by magnetic attracting force of a permanent magnet or electromagnet.
The impact printers can be divided, according to the type of the wire-dot print head, into the plunger type, the spring-charge type, and the clapper type.
The spring-charge type is of a structure in which an armature to which a print wire is fixed is supported to a plate spring in such a way that it can be swung, and the armature is attracted to a core by a permanent magnet overcoming the resilient force of the plate spring, and for printing, the coil wound on the core is energized to generate a magnetic flux in opposition to the magnetic flux from the permanent magnet to release the armature.
In the clapper type, the coil is energized for printing to generate a magnetic flux thereby to attract the plate spring to the coil and printing is performed by the attracting force.
FIG. 6 is a cross sectional view showing the above-described prior-art wire-dot print head.
In such figure, provided between a guide frame 1 and a cap 2 are a base plate 3, a permanent magnet 4, an upright support 5, a spacer 6, a plate spring 7 and a yoke 8 which are stacked successively with each other and clamped by a clamp 9.
Provided on a flexible part of the plate spring 7 is an armature 10. Fixed to the tip of the armature 10 is a base part of a print wire 11, tip of which is guided by a guide 1a to project toward a platen.
A core 12 is provided in the center of the base plate 3. 14 is a circuit board for energizing a coil 13. 15 is a spacing sheet for positioning the board 14. 16 is a temperature-detecting thermistor. 17 is a filler having a high thermal conductivity and covering the coil 13 and the thermistor 16.
With the above structure, a magnetic circuit is formed whereby magnetic flux from the permanent magnet 4 is passed through the upright support 5, the spacer 6, the yoke 8, the armature 10, the core 12 and the base 3 and returns to the permanent magnet 4. Because of this magnetic circuit, the armature 10 is attracted to the core 12 into a biased state to store distortion energy in the plate spring 7.
In this biased state, if the coil 13 is energized to generate a magnetic flux in opposition to the magnetic circuit, the force for attracting the armature 10 is reduced.
For this reason, the distortion energy stored in the plate spring 7 is released and the plate spring 7 is restored, so that the print wire 11 fixed to the tip of the armature 10 projects from the guide 1a and presses an ink ribbon and a print medium against the platen.
In this way, characters and graphic patterns are printed.
By energizing the coil 13 during printing, generated heat is transmitted to the thermistor 16 through the filler 17 made of epoxy resin or the like which has a high heat conductivity, and the temperature within the wire-dot print head is supervised and the coil 13 is controlled below its maximum operating temperature.
For the permanent magnet 4, materials of the samarium-cobalt type having a high energy product (BH product) and low temperature coefficient of magnetic flux density are frequently employed.
With the above-described prior-art wire-dot print head, instead of the permanent magnet of the samarium-cobalt type containing rare, samarium and cobalt as main constituents, permanent magnets of the neodyminum type are used to increase the printing speed and to lower the price of the printer.
In such a case, the temperature coefficient of the residual magnetic flux density of the permanent magnet is four to five times greater than that of the permanent magnet of the samarium-cobalt type, and the attracting force generated by the permanent magnet 4 varies due to the heat generated by the coil 13 within the wire-dot print head. Moreover, in the worst case, the plate spring 7 cannot be attracted.
FIG. 7 shows the relationship between the attraction stroke, and the spring force and the attracting force in the prior-art wire-dot print head. Attracting force curves both at a high temperature and at a low temperature are shown.
The attracting force F.sub.0 at the fully attracted point of the armature 10 decreases with a rise in temperature, and keeps decreasing to F.sub.1 for the highest operating temperature of the wire-dot print head. At this temperature, the attracting force may become smaller than the spring force and failure of attraction of the plate spring may occur.
Where the printing speed is increased, the weight of the wire-dot print head is reduced, heat generated from the coil 13 during printing increased, and the heat radiation capacity is reduced. The temperature rises more quickly and reaches the maximum operating temperature in a shorter time. Because heat control is performed to suppress the temperature rise, printing is suspended or one-way printing is performed or some other action to reduce the duty ratio is taken. As a result, the printing speed (throughput) is lowered.
Another problem relates to magnetic interferences between adjacent cores. FIG. 8 is a developed view of the core for explaining the magnetic interference in the prior-art wire-dot print head.
In such figure, 12a, 12b and 12c are cores provided in juxtaposition. 13 is a coil. 7 is a plate spring having a print wire. When the coil 13 is not energized, the plate spring 7 is attracted by the magnetic flux, shown by solid lines, generated by the permanent magnet. When a drive current is made to flow through the coils 13 on the core 12a and the core 12c, a magnetic flux, shown by dotted lines, is generated and part of this passes through the core 12b.
The direction of this leakage flux is identical to the direction of the magnetic flux for attracting the plate spring 7, so the attracting force is increased. Because of the effect of the magnetic flux, when a plurality of print wires 11 are driven simultaneously, the printing force is lowered.
To compensate for this, control is made whereby the drive time for which the drive current is made to flow through the coils 13 is varied with the number of the print wires simultaneously driven, and the drive time is lengthened with the number of the driven print wires. In such case, heat generated from the coils 13 is increased, so the print duty is further lowered.